The present application claims priority to Japanese Application No. P2000-128998 filed Apr. 25, 2000, which application is incorporated herein by reference to the extent permitted by law.
This invention relates to a positive electrode and a non-aqueous electrolyte cell employing a lithium compound oxide as a positive electrode active material.
Recently, electronic equipment, such as video cameras or headphone type stereo devices, are being rapidly improved in performance and reduced in size, so that an increasing demand is raised towards a higher capacity of the secondary cell as a power source of these electronic equipment. As the secondary cells, lead secondary cells, nickel-cadmium secondary cells and nickel hydrogen cells have so far been used. A non-aqueous electrolyte secondary cell, employing a carbonaceous material and a lithium cobalt oxide (LiCoO2) as negative electrode active material and positive electrode active material, respectively, resorts to doping/undoping of lithium to suppress dendritic growth or pulverization of lithium, thus achieving superior cyclic useful life as well as a high energy density and a high capacity. As the positive electrode active material for this lithium secondary cell, LiNiO2 having the same spatial group R3m/layered structure as that of LiCoO2 and LiMn2O4 having the normal spinel structure and the spatial group Fd3m, have been put to practical use besides LiCoO2.
However, the lithium ion secondary cell, employing the above-mentioned positive electrode active material, is more costly than the conventional secondary cell, mainly due to the cost involved in the positive electrode active material. Since this is ascribable to the fact that transition metals, as constituent elements, are rare. It is therefore desirable to use a material which is based on more abundant and inexpensive elements, such as iron.
On the other hand, the conventional positive electrode active material is problematic in general in operational stability. This is caused by high voltage and consequent high reactivity with the electrolytic solution and by instabilities in the crystalline structure. Thus, it is a frequent occurrence that sufficient stability is not displayed in high-temperature cyclic characteristics, storage characteristics or in self-discharge performance.
The present inventors were the first to win success with an iron compound in controlling various physical properties required of the positive electrode for the lithium cell, and in realizing the energy density equivalent to that of the conventional material, such as LiCoO2, LiNiO2 or LiMn2O4, through optimization of the synthesis process of an iron-based material LiFePO4 Moreover, as a result of our eager researches, the present inventors have found that this material is an ideal material, insofar as cost and stability are concerned, in that the material is excellent in high temperature stability, and in that it is substantially free from cyclic or storage deterioration or self-discharge even at elevated temperatures of 80xc2x0 C.
However, the cell displays extremely flat charging/discharging characteristics at a generated voltage of 3.4V. The cell has a somewhat low voltage and different charging/discharging curve, in comparison with the moderate charging/discharging characteristics from 4.0 to 3.5V of conventional materials, such that LiFePO4, if used alone, cannot be made compatible with widely used lithium ion secondary cell.
The conventional lithium ion secondary cell suffers not only from the above-mentioned cost and operational stability, but also from the drawback that, if overcharged, charging/discharging characteristics are deteriorated. That is, if the cell is open-circuited when an electronic equipment employing the cell falls into disorder or if a cut-off voltage is not set in the electronic equipment, with the discharging voltage being 0V, the open-circuit voltage is not restored, such that, if the cell is subsequently charged or discharged, the cell capacity is lowered appreciably. The charging/discharging characteristics of the secondary cell in case it has been over-discharged to 0V are crucial for practical use of the secondary cell, such that measures against deterioration of the charging/discharging characteristics are indispensable.
The reason for deterioration in over-discharging and short useful life is that the potential of copper as the negative electrode current collector is pulled during the terminal process of the over-discharging by the operating potential of the positive electrode which is as high as 3.5 V to exceed the voltage of precipitation dissolution of copper of 3.45V, thus inducing the dissolution reaction of copper, as described in JP Patent No. 2797390.
The present invention has been proposed with the above-described status of the prior art in mind. Thus, it is an object of the present invention to provide a positive electrode with which it is possible to assure compatibility of a cell employing the positive electrode with a conventional lithium ion cell, an energy density of the cell equivalent to that of the conventional lithium ion cell, an appreciably improved operational stability under special conditions, such as elevated temperatures, and superior performance against over-discharging, as well as to construct a lithium ion cell less costly than the conventional lithium ion cell. It is another object of the present invention to provide a non-aqueous electrolyte cell employing the positive electrode.
A positive electrode according to the present invention includes a layer of a positive electrode active material is formed on a positive electrode current collector, and wherein the layer of the positive electrode active material contains, as a positive electrode active material, a composite product of a first lithium compound represented by the general formula LixMyPO4, where 0 less than x less than 2, 0.8 less than y less than 1.2 and M contains Fe, and a second lithium compound having a potential holder than the potential of the first lithium compound.
The positive electrode according to the present invention uses the composite material comprised of the first lithium compound and the second lithium compound, as the positive electrode active material, so that, during charging/discharging, reaction takes place continuously between the first and second lithium compounds. If this positive electrode is used as the cell, it becomes possible to suppress discontinuous voltage changes during over-charging and charging/discharging to a minimum to assure stable charging/discharging characteristics.
A non-aqueous electrolyte cell according to the present invention includes a positive electrode including a positive electrode current collector carrying a layer of a positive electrode active material thereon, a negative electrode including a negative electrode active material carrying a layer of a negative electrode active material thereon and a non-aqueous electrolyte interposed between the positive electrode and the negative electrode, wherein the layer of the positive electrode active material contains, as a positive electrode active material, a composite product of a first lithium compound represented by the general formula LixMyPO4, where 0 less than x less than 2, 0.8  less than y less than 1.2 and M contains Fe, and a second lithium compound having a potential holder than the potential of the first lithium compound.
The positive non-aqueous electrolyte cell according to the present invention uses the composite material comprised of the first lithium compound and the second lithium compound, as the positive electrode active material, so that, during charging/discharging, reaction takes place continuously between the first and second lithium compounds. So, it becomes possible to suppress discontinuous voltage changes during over-charging and charging/discharging to a minimum to assure stable charging/discharging characteristics.
According to the present invention, a non-aqueous electrolyte cell having superior charging/discharging characteristics and cyclic characteristics may be realized by employing a compound system comprised of the first and second lithium compounds having respective different potentials.